Dynamic Spatial Index Remapping for Optimal Aggregate Performance

ABSTRACT

By mapping leaf nodes of a spatial index to processing elements, efficient distribution of workload in an image processing system may be achieved. In addition, processing elements may use a thread table to redistribute workload from processing elements which are experiencing an increased workload to processing elements which may be idle. Furthermore, the workload experienced by processing elements may be monitored in order to determine if workload is balanced. Periodically the leaf nodes for which processing elements are responsible may be remapped in response to a detected imbalance in workload. By monitoring the workload experienced by the processing elements and remapping leaf nodes to different processing elements in response to unbalanced workload, efficient distribution of workload may be maintained. Efficient distribution of workload may improve the performance of the image processing system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to the field of image processing.

2. Description of the Related Art

The process of rendering two-dimensional images from three-dimensional scenes is commonly referred to as image processing. As the modern computer industry evolves image processing evolves as well. One particular goal in the evolution of image processing is to make two-dimensional simulations or renditions of three-dimensional scenes as realistic as possible. One limitation of rendering realistic images is that modern monitors display images through the use of pixels.

A pixel is the smallest area of space which can be illuminated on a monitor. Most modern computer monitors will use a combination of hundreds of thousands or millions of pixels to compose the entire display or rendered scene. The individual pixels are arranged in a grid pattern and collectively cover the entire viewing area of the monitor. Each individual pixel may be illuminated to render a final picture for viewing.

One technique for rendering a real world three-dimensional scene onto a two-dimensional monitor using pixels is called rasterization. Rasterization is the process of taking a two-dimensional image represented in vector format (mathematical representations of geometric objects within a scene) and converting the image into individual pixels for display on the monitor. Rasterization is effective at rendering graphics quickly and using relatively low amounts of computational power; however, rasterization suffers from some drawbacks. For example, rasterization often suffers from a lack of realism because it is not based on the physical properties of light, rather rasterization is based on the shape of three-dimensional geometric objects in a scene projected onto a two dimensional plane. Furthermore, the computational power required to render a scene with rasterization scales directly with an increase in the complexity of the scene to be rendered. As image processing becomes more realistic, rendered scenes also become more complex. Therefore, rasterization suffers as image processing evolves, because rasterization scales directly with complexity.

Another technique for rendering a real world three-dimensional scene onto a two-dimensional monitor using pixels is called ray tracing. The ray tracing technique traces the propagation of imaginary rays, rays which behave similar to rays of light, into a three-dimensional scene which is to be rendered onto a computer screen. The rays originate from the eye(s) of a viewer sitting behind the computer screen and traverse through pixels, which make up the computer screen, towards the three-dimensional scene. Each traced ray proceeds into the scene and may intersect with objects within the scene. If a ray intersects an object within the scene, properties of the object and several other contributing factors are used to calculate the amount of color and light, or lack thereof, the ray is exposed to. These calculations are then used to determine the final color of the pixel through which the traced ray passed.

The process of tracing rays is carried out many times for a single scene. For example, a single ray may be traced for each pixel in the display. Once a sufficient number of rays have been traced to determine the color of all of the pixels which make up the two-dimensional display of the computer screen, the two dimensional synthesis of the three-dimensional scene can be displayed on the computer screen to the viewer.

Ray tracing typically renders real world three-dimensional scenes with more realism than rasterization. This is partially due to the fact that ray tracing simulates how light travels and behaves in a real world environment, rather than simply projecting a three-dimensional shape onto a two dimensional plane as is done with rasterization. Therefore, graphics rendered using ray tracing more accurately depict on a monitor what our eyes are accustomed to seeing in the real world.

Furthermore, ray tracing also handles increases in scene complexity better than rasterization as scenes become more complex. Ray tracing scales logarithmically with scene complexity. This is due to the fact that the same number of rays may be cast into a scene, even if the scene becomes more complex. Therefore, ray tracing does not suffer in terms of computational power requirements as scenes become more complex as rasterization does.

One major drawback of ray tracing is the large number of calculations, and thus processing power, required to render scenes. This leads to problems when fast rendering is needed. For example, when an image processing system is to render graphics for animation purposes such as in a game console. Due to the increased computational requirements for ray tracing it is difficult to render animation quickly enough to seem realistic (realistic animation is approximately twenty to twenty-four frames per second).

Therefore, there exists a need for more efficient techniques and devices to perform ray tracing.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally provide methods and apparatus for performing ray tracing.

According to one embodiment of the invention, a method of distributing workload in a ray tracing image processing system is provided. The method generally comprising: providing a spatial index having internal nodes and leaf nodes defining bounded volumes of a three dimensional scene; dividing the leaf nodes into at least a first group and a second group, wherein a first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group, and wherein a second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; tracing rays to render a two dimensional image from the three dimensional scene; monitoring at least a workload experienced by the first processing element or a workload experienced by the second processing element; and re-dividing the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.

According to another embodiment of the invention a computer readable medium computer readable medium containing a program is provided. The program which, when executed, performs an operation generally comprising: providing a spatial index having internal nodes and leaf nodes defining bounded volumes of a three dimensional scene; dividing the leaf nodes into at least a first group and a second group, wherein a first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group, and wherein a second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; tracing rays to render a two dimensional image from the three dimensional scene; monitoring at least a workload experienced by the first processing element or a workload experienced by the second processing element; and re-dividing the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.

According to another embodiment of the invention an image processing system is provided. The image processing system generally comprising: a spatial index having internal nodes and leaf nodes defining bounding volumes of a three-dimensional scene; a first processing element; and a second processing element; and wherein the image processing system is configured to divide the leaf nodes of the spatial index into at least a first group and a second group, and trace rays to render a two dimensional image from the three dimensional scene; wherein the first processing element configured to perform operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group; wherein the second processing element configured to perform operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; and wherein the image processing system is further configured to monitor at least a workload experienced by the first processing element or a workload experienced by the second processing element, and re-divide the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an exemplary computer processor, according to one embodiment of the invention.

FIG. 2 illustrates a multiple-core processing element network, according to one embodiment of the invention.

FIGS. 3A-3C are block diagrams illustrating aspects of memory inboxes according to one embodiments of the invention.

FIG. 4 is an exemplary three-dimensional scene to be rendered by an image processing system, according to one embodiment of the invention.

FIGS. 5A-5C illustrate a two dimensional space to be rendered by an image processing system and a corresponding spatial index created by an image processing system, according to one embodiment of the invention.

FIG. 6 is a flowchart illustrating a method of performing ray tracing, according to one embodiment of the invention.

FIG. 7 is an exemplary three-dimensional space to be rendered by an image processing system, according to one embodiment of the invention.

FIGS. 8A-8D illustrate an exemplary method of performing ray tracing, according to one embodiment of the invention.

FIGS. 9 and 13 illustrate an exemplary spatial index, according to one embodiment of the invention.

FIGS. 10 and 12 are flowcharts illustrating exemplary methods of performing ray tracing, according to one embodiment of the invention.

FIG. 11 is an exemplary thread table, according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention provide methods and apparatus to distribute workload amongst processing elements in a ray tracing image processing system. According to one embodiment of the invention, operations relating to rays which are traced to leaf nodes of a spatial index may be mapped or assigned to a plurality of processing elements. The processing elements responsible for the operations may monitor their workload, and if the workload exceeds a threshold the processing elements may transfer the operations relating to the rays to other idle processing elements. Furthermore, the image processing system may monitor the workload experienced by the processing elements. In response to an unbalanced workload amongst the processing elements, the image processing system may remap or reassign the operations to different processing elements.

In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, in various embodiments the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

One embodiment of the invention is implemented as a program product for use with a computer system such as, for example, the image processing system described below. The program(s) of the program product defines functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable media. Illustrative computer-readable media include, but are not limited to: (i) information permanently stored on non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive); (ii) alterable information stored on writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive); and (iii) information conveyed to a computer by a communications medium, such as through a computer or telephone network, including wireless communications. The latter embodiment specifically includes information downloaded from the Internet and other networks. Such computer-readable media, when carrying computer-readable instructions that direct the functions of the present invention, represent embodiments of the present invention.

In general, the routines executed to implement the embodiments of the invention, may be part of an operating system or a specific application, component, program, module, object, or sequence of instructions. The computer program of the present invention typically is comprised of a multitude of instructions that will be translated by the native computer into a machine-readable format and hence executable instructions. Also, programs are comprised of variables and data structures that either reside locally to the program or are found in memory or on storage devices. In addition, various programs described hereinafter may be identified based upon the application for which they are implemented in a specific embodiment of the invention. However, it should be appreciated that any particular program nomenclature that follows is used merely for convenience, and thus the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature.

An Exemplary Multiple Core Processing Element

FIG. 1 illustrates a multiple core processing element 100, according to one embodiment of the invention. The multiple core processing element 100 includes a plurality of basic throughput engines 105 (BTEs). A BTE 105 may contain a plurality of processing threads and a core cache (e.g., an L1 cache). The processing threads located within each BTE may have access to a shared multiple core processing element memory cache 110 (e.g., a shared L2 cache).

The BTEs 105 may also have access to a plurality of inboxes 115. The inboxes 115, described further below with regards to FIG. 3, may be memory mapped address space. The inboxes 115 may be mapped to the processing threads located within each of the BTEs 105. Each thread located within the BTEs may have a memory mapped inbox and access to all of the other memory mapped inboxes 115. The inboxes 115 make up a low-latency and high-bandwidth communications network used by the BTEs 105.

The BTEs may use the inboxes 115 as a network to communicate with each other and redistribute data processing work amongst the BTEs. For some embodiments, separate outboxes may be used in the communications network, for example, to receive the results of processing by BTEs 105. For other embodiments, inboxes 115 may also serve as outboxes, for example, with one BTE 105 writing the results of a processing function directly to the inbox of another BTE 105 that will use the results.

The aggregate performance of an image processing system may be tied to how well the BTEs can partition and redistribute work. The network of inboxes 115 may be used to collect and distribute work to other BTEs without corrupting the shared multiple core processing element cache 110 with BTE communication data packets that have no frame to frame coherency. An image processing system which can render many millions of triangles per frame may include many BTEs 105 connected in this manner.

In one embodiment of the invention, the threads of one BTE 105 may be assigned to a workload manager. An image processing system may use various software and hardware components to render a two dimensional image from a three-dimensional scene. As described further below with regards to FIG. 6, according to one embodiment of the invention, an image processing system may use a workload manager to traverse a spatial index with a ray issued by the image processing system. A spatial index, as described further below with regards to FIG. 4, may be implemented as a tree type data structure used to partition a relatively large three-dimensional scene into smaller bounding volumes. An image processing system using a ray tracing methodology for image processing may use a spatial index to quickly determine ray-bounding volume intersections. In one embodiment of the invention, the workload manager may perform ray-bounding volume intersection tests by using the spatial index.

In one embodiment of the invention, other threads of the multiple core processing element BTEs 105 on the multiple core processing element 100 may be vector throughput engines. After a workload manager determines a ray-bounding volume intersection, the workload manager may issue (send), via the inboxes 115, the ray to one of a plurality of vector throughput engines. According to one embodiment of the invention, and described further below with regards to FIG. 6, the vector throughput engines may then determine if the ray intersects a primitive contained within the bounding volume. The vector throughput engines may also perform operations relating to determining the color of the pixel through which the ray passed.

FIG. 2 illustrates a network of multiple core processing elements 200, according to one embodiment of the invention. FIG. 2 also illustrates one embodiment of the invention where the threads of one of the BTEs of the multiple core processing element 100 is a workload manager 205. Each multiple core processing element 220 _(1-N) in the network of multiple core processing elements 200 may contain one workload manager 205 _(1-N), according to one embodiment of the invention. Each processor 220 in the network of multiple core processing elements 200 may also contain a plurality of vector throughput engines 210, according to one embodiment of the invention.

The workload managers 220 _(1-N) may use a high speed bus 225 to communicate with other workload managers 220 _(1-N) and/or vector throughput engines 210 of other multiple core processing elements 220, according to one embodiment of the invention. Each of the vector throughput engines 210 may use the high speed bus 225 to communicate with other vector throughput engines 210 or the workload managers 205. The workload manager processors 205 may use the high speed bus 225 to collect and distribute image processing related tasks to other workload manager processors 205, and/or distribute tasks to other vector throughput engines 210. The use of a high speed bus 225 may allow the workload managers 205 _(1-N) to communicate without affecting the caches 230 with data packets related to workload manager 205 communications.

Low-Latency High-Bandwidth Communications Network

As described above, the aggregate performance of an image processing system may be tied to how well the BTEs can partition and redistribute work. According to one embodiment of the invention, memory space within a cache, referred to as a memory inbox, may be used to distribute work to a single processor thread. In an image processing system using a plurality of processors each having a plurality of threads, the collection of inboxes together may be referred to as a low-latency high-bandwidth communications network.

In multithreading processor such as a BTE 105, a memory inbox may be assigned to a given thread (referred to herein as the owner thread). In one embodiment of the invention, the memory space for the inbox may be allocated from the shared memory cache 110 exclusively to the owner thread. By exclusively assigning the memory space in a cache to the owner thread, the owner thread may maintain enough memory space to cache its own instructions and data without other having other competing threads displace the owner thread's instructions and data. Thus, the memory inbox may improve execution of the owner thread by maintaining the owner thread's data and instructions in the assigned inbox portion of the cache and reducing the possibility of stalling the owner thread while data and instructions for the owner thread are retrieved from higher levels of memory. Furthermore, by assigning the memory space in a cache to the owner thread, data or instructions intended for the targeted thread may be stored only in an inbox allocated to the thread. Thus, data or instructions intended for the targeted thread are not stored throughout the shared memory cache 110, rather only in the inbox allocated to the targeted thread.

Furthermore, the inbox memory may be used by other threads to efficiently communicate with the owner thread. For example, where another thread has data and/or instructions which are to be provided to the owner thread for an inbox, the other thread may send the data and/or instructions to the inbox where the data and/or instructions may be retrieved by the owner thread. Similarly, in some cases, the owner thread may use the inbox as an outbox to communicate information with other threads. For example, to communicate the information with another thread, the owner thread may place the information in the inbox and send a notification to the other thread indicating the location of the data and/or instructions, thereby allowing the other thread to retrieve the information. Optionally, the owner thread may provide the information directly to the inbox of the other thread. Thus, the inbox memory may be used to simplify communication between a sending and a receiving thread while preventing displacement of data and/or instructions being used by other threads.

FIG. 3A is a block diagram of memory inboxes 302 . . . 318 in a multi-core processor element 100 according to one embodiment of the invention. The depiction of the memory inboxes 302 . . . 318 is intended to be a conceptual view and therefore is not limited to any particular physical configuration. As depicted, threads (e.g., threads T0-T7) executing in each core (e.g., the BTEs 105) may have access to the shared L2 cache 110 via a shared L2 cache interface 322. Furthermore, the L2 cache interface 322 may also be used by the threads T0 . . . T7 to access the corresponding memory inboxes 302 . . . 318. As described above, in some cases, each inbox 302 . . . 318 may be assigned to a corresponding thread T0-T7. Thus, Inbox 0 302 may be assigned to thread T0 and so on. As described below, by assigning a given inbox to a given thread, access to the assigned inbox may be unrestricted with respect to the owner thread while access by other threads may be restricted. Exemplary restrictions are described below in greater detail.

FIG. 3B is a block diagram depicting the path of data from memory inboxes (e.g., inboxes 302 . . . 308) and the shared L2 cache 110 transmitted to and from a processing core (e.g., BTE 105). As described above, both the memory inboxes 302 . . . 308 and the shared L2 cache 110 may be accessed via the shared L2 cache interface 322. Where a thread being executed in the BTE 105 retrieves data from an inbox 302 . . . 308 or from the shared L2 cache 110, the retrieved data may be placed in the L1 cache 312 for the BTE 105. Instructions for the thread may be issued from an issue unit 332. In some cases, the BTE 105 may be configured to execute multiple threads concurrently. Thus, the issue unit 332 may be configured to issue instructions for multiple threads. In some cases, the BTE 105 may provide multiple execution units 334 . . . 338 which may be used to concurrently execute threads in the BTE 105. The execution units 334 . . . 338 may include a fixed point execution unit 334, a floating point execution unit 336, and a branch execution unit 338.

In some cases, a thread may update or produce data which is to be accessed later (e.g., by the same thread or by another thread). Where the updated data is to be accessed later, the thread may place the updated data in an L1 cache 312. Furthermore, where desired, the updated data may also be placed in the L2 cache 110 or in an inbox 302 . . . 308 for the updating thread via the shared L2 cache interface 322. In some cases, as described above, direct access to a given inbox (e.g., inbox 0 302) via the shared L2 cache interface 322 may be limited to the thread (e.g., thread T0) which owns the given inbox.

In one embodiment of the invention, memory space within a memory inbox may be mapped to a global memory address (e.g., all levels of memory including the L1 cache 312, L2 cache 110, and main memory as well as all threads may use the same global memory address to access a given memory inbox). Thus, in one embodiment of the invention, to access the inbox memory space, the owner thread may merely read or write the desired information to a global memory address corresponding to the inbox memory space. A thread which does not own the memory inbox and which attempts to directly access the inbox via the global memory address, may have access to the inbox denied. Other forms of access may instead be provided to other non-owning threads, e.g., via packetized messages sent to the inbox.

Also, in one embodiment of the invention, information being stored in a memory inbox may not be cacheable. For example, while information in the L1 cache 312, L2 cache 110, and other memory level may be automatically cached by the multi core processing element 100 such that information requested from a given memory address may be automatically fetched from main memory and maintained in one of the cache levels 312, 110 while being accessed. In contrast, the globally addressable memory in a given inbox may only be located in the inbox and may not be moved between different levels of the memory hierarchy (e.g., the main memory, the shared L2 cache memory 110, or the L1 cache memory) without being copied to a new address space outside of the inbox. Thus, accesses to an inbox by an owner thread may be performed quickly and directly to the inbox memory without waiting for information to be fetched from another level of the memory hierarchy and/or translated during fetching. The non-cacheability of inbox memory may also apply with respect to packetized access of the inbox described below. Furthermore, in an alternate embodiment of the invention, information stored in the inbox may be cached in other levels of the memory hierarchy.

Assignment of Memory Inboxes

In one embodiment of the invention, memory inboxes may be provided from the shared memory cache 110 (e.g., a portion of the L2 cache 110 may be reserved for the inbox memory 115). FIG. 3C is a block diagram depicting inbox memory 115 partitioned from the shared L2 cache 110 according to one embodiment of the invention.

As depicted, the size and location of each inbox 302, 304, etc. may be controlled by inbox control registers 340. The status of each inbox 302, 304, etc. (e.g., enabled or disabled) may be indicated and/or modified via inbox status registers 362. In one embodiment, access to the inbox control registers 340 may be unrestricted. Optionally, in some cases, access to the inbox control registers may be limited, for example, to a subset of approved threads (e.g., the owner thread, a parent of the owner thread, a specially designated control thread, and/or an operating system kernel thread). In one embodiment, the inbox control registers 340 may include a start address register 342, 348 . . . 354, a size register 344, 350 . . . 356, and an owner thread identification register 346, 352 . . . 358.

In one embodiment, the start address registers 342, 348 . . . 354 may indicate a start address for each inbox 302, 304, etc. The size registers 344, 350 . . . 358 may indicate the size of a corresponding inbox 302, 304, etc. The memory space for an inbox may thus occupy each address beginning from the corresponding start address and ranging through the indicated size of the inbox. The size may be indicated in any manner, for example, as an absolute size in bytes or as an integer multiple of a fixed size (e.g., the size in the size registers 344, 350 . . . 358 may indicate the size in kilobytes).

In one embodiment, the owner thread identification register 346, 352 . . . 358 may identify which thread (e.g., thread T0, T1 . . . TN) owns a given inbox 302, 304, etc. While depicted with respect to threads and corresponding inboxes 1, 2 . . . N, embodiment of the invention may be used with any type of thread and/or inbox identifier (e.g., a number, an address, etc.). In one embodiment of the invention, the inbox identifier register may be used to restrict direct access to memory addresses within the corresponding inbox to the owner thread. In some cases, direct access may also be allowed by a limited selection of other threads, such as, for example, a parent thread of the owner thread, a specified control thread, and/or an operating system kernel thread. In one embodiment, access control circuitry 360 may be used to provide the restricted access.

By assigning portions of the shared memory cache 110 to the inboxes a low-latency high-bandwidth communications network may be formed. The remaining portion of the shared memory cache 110 may remain unassigned and, thus, available to store information which does not relate to communications between processing threads. The remaining portion of the shared memory cache 110 may be used to store geometry and data structures which are used by the image processing system to perform ray tracing (described further below with respect to FIG. 5).

A benefit of using only the inboxes for communications between processing threads and using the remaining portion of the shared memory cache 110 to store geometry and data structures is that no matter how much communications related information is passed through the inboxes, it will not consume the entire memory cache. Thus, as will be described further below, communications related information will not displace the geometry and data structures stored within the remaining portion of the shared memory cache 100. Therefore, data which is likely to be reused when tracing subsequent rays or rendering subsequent frames (object geometry and data structures) may remain in the cache, while data which is unlikely to be reused when tracing subsequent rays or rendering subsequent frames (data processing work) will not remain in the cache.

An Exemplary Three-Dimensional Scene

FIG. 4 is an exemplary three-dimensional scene 405 to be rendered by an image processing system. Within the three-dimensional scene 405 may be objects 420. The objects 420 in FIG. 4 are of different geometric shapes. Although only four objects 420 are illustrated in FIG. 4, the number of objects in a typical three-dimensional scene may be more or less. Commonly, three-dimensional scenes will have many more objects than illustrated in FIG. 4.

As can be seen in FIG. 4 the objects are of varying geometric shape and size. For example, one object in FIG. 4 is a pyramid 420 _(A). Other objects in FIG. 4 are boxes 420 _(B-D). In many modern image processing systems objects are often broken up into smaller geometric shapes (e.g., squares, circles, triangles, etc.). The larger objects are then represented by a number of the smaller simple geometric shapes. These smaller geometric shapes are often referred to as primitives.

Also illustrated in the scene 405 are light sources 425 _(A-B). The light sources may illuminate the objects 420 located within the scene 405. Furthermore, depending on the location of the light sources 425 and the objects 420 within the scene 405, the light sources may cause shadows to be cast onto objects within the scene 405.

The three-dimensional scene 405 may be rendered into a two-dimensional picture by an image processing system. The image processing system may also cause the two-dimensional picture to be displayed on a monitor 410. The monitor 410 may use many pixels 430 of different colors to render the final two-dimensional picture.

One method used by image processing systems to rendering a three-dimensional scene 420 into a two dimensional picture is called ray tracing. Ray tracing is accomplished by the image processing system “issuing” or “shooting” rays from the perspective of a viewer 415 into the three-dimensional scene 420. The rays have properties and behavior similar to light rays.

One ray 440, that originates at the position of the viewer 415 and traverses through the three-dimensional scene 405, can be seen in FIG. 4. As the ray 440 traverses from the viewer 415 to the three-dimensional scene 405, the ray 440 passes through a plane where the final two-dimensional picture will be rendered by the image processing system. In FIG. 4 this plane is represented by the monitor 410. The point the ray 440 passes through the plane, or monitor 410, is represented by a pixel 435.

As briefly discussed earlier, most image processing systems use a grid 430 of thousands (if not millions) of pixels to render the final scene on the monitor 410. Each individual pixel may display a different color to render the final composite two-dimensional picture on the monitor 410. An image processing system using a ray tracing image processing methodology to render a two dimensional picture from a three-dimensional scene will calculate the colors that the issued ray or rays encounters in the three-dimensional scene. The image processing scene will then assign the colors encountered by the ray to the pixel through which the ray passed on its way from the viewer to the three-dimensional scene.

The number of rays issued per pixel may vary. Some pixels may have many rays issued for a particular scene to be rendered. In which case the final color of the pixel is determined by the each color contribution from all of the rays that were issued for the pixel. Other pixels may only have a single ray issued to determine the resulting color of the pixel in the two-dimensional picture. Some pixels may not have any rays issued by the image processing system, in which case their color may be determined, approximated or assigned by algorithms within the image processing system.

To determine the final color of the pixel 435 in the two dimensional picture, the image processing system must determine if the ray 440 intersects an object within the scene. If the ray does not intersect an object within the scene it may be assigned a default background color (e.g., blue or black, representing the day or night sky). Conversely, as the ray 440 traverses through the three-dimensional scene the ray 440 may strike objects. As the rays strike objects within the scene the color of the object may be assigned the pixel through which the ray passes. However, the color of the object must be determined before it is assigned to the pixel.

Many factors may contribute to the color of the object struck by the original ray 440. For example, light sources within the three-dimensional scene may illuminate the object. Furthermore, physical properties of the object may contribute to the color of the object. For example, if the object is reflective or transparent, other non-light source objects may then contribute to the color of the object.

In order to determine the effects from other objects within the three-dimensional scene, secondary rays may be issued from the point where the original ray 440 intersected the object. For example, one type of secondary ray may be a shadow ray. A shadow ray may be used to determine the contribution of light to the point where the original ray 440 intersected the object. Another type of secondary ray may be a transmitted ray. A transmitted ray may be used to determine what color or light may be transmitted through the body of the object. Furthermore, a third type of secondary ray may be a reflected ray. A reflected ray may be used to determine what color or light is reflected onto the object.

As noted above, one type of secondary ray may be a shadow ray. Each shadow ray may be traced from the point of intersection of the original ray and the object, to a light source within the three-dimensional scene 405. If the ray reaches the light source without encountering another object before the ray reaches the light source, then the light source will illuminate the object struck by the original ray at the point where the original ray struck the object.

For example, shadow ray 441 _(A) may be issued from the point where original ray 440 intersected the object 420 _(A), and may traverse in a direction towards the light source 425 _(A). The shadow ray 441 _(A) reaches the light source 425 _(A) without encountering any other objects 420 within the scene 405. Therefore, the light source 425 _(A) will illuminate the object 420 _(A) at the point where the original ray 440 intersected the object 420 _(A).

Other shadow rays may have their path between the point where the original ray struck the object and the light source blocked by another object within the three-dimensional scene. If the object obstructing the path between the point on the object the original ray struck and the light source is opaque, then the light source will not illuminate the object at the point where the original ray struck the object. Thus, the light source may not contribute to the color of the original ray and consequently neither to the color of the pixel to be rendered in the two-dimensional picture. However, if the object is translucent or transparent, then the light source may illuminate the object at the point where the original ray struck the object.

For example, shadow ray 441 _(B) may be issued from the point where the original ray 440 intersected with the object 420 _(A), and may traverse in a direction towards the light source 425 _(B). In this example, the path of the shadow ray 441 _(B) is blocked by an object 420 _(D). If the object 420 _(D) is opaque, then the light source 425 _(B) will not illuminate the object 420 _(A) at the point where the original ray 440 intersected the object 420 _(A). However, if the object 420 _(D) which the shadow ray is translucent or transparent the light source 425 _(B) may illuminate the object 420 _(A) at the point where the original ray 440 intersected the object 420 _(A).

Another type of secondary ray is a transmitted ray. A transmitted ray may be issued by the image processing system if the object with which the original ray intersected has transparent or translucent properties (e.g., glass). A transmitted ray traverses through the object at an angle relative to the angle at which the original ray struck the object. For example, transmitted ray 444 is seen traversing through the object 420 _(A) which the original ray 440 intersected.

Another type of secondary ray is a reflected ray. If the object with which the original ray intersected has reflective properties (e.g. a metal finish), then a reflected ray will be issued by the image processing system to determine what color or light may be reflected by the object. Reflected rays traverse away from the object at an angle relative to the angle at which the original ray intersected the object. For example, reflected ray 443 may be issued by the image processing system to determine what color or light may be reflected by the object 420 _(A) which the original ray 440 intersected.

The total contribution of color and light of all secondary rays (e.g., shadow rays, transmitted rays, reflected rays, etc.) will result in the final color of the pixel through which the original ray passed.

An Exemplary KD-Tree

One problem encountered when performing ray tracing is determining quickly and efficiently if an issued ray intersects any objects within the scene to be rendered. One methodology known by those of ordinary skill in the art to make the ray intersection determination more efficient is to use a spatial index. A spatial index divides a three-dimensional scene or world into smaller volumes (smaller relative to the entire three-dimensional scene) which may or may not contain primitives. An image processing system can then use the known boundaries of these smaller volumes to determine if a ray may intersect primitives contained within the smaller volumes. If a ray does intersect a volume containing primitives, then a ray intersection test can be run using the trajectory of the ray against the known location and dimensions of the primitives contained within that volume. If a ray does not intersect a particular volume then there is no need to run ray-primitive intersection tests against the primitives contained within that volume. Furthermore, if a ray intersects a bounding volume which does not contain primitives then there is no need to run ray-primitive intersections tests against that bounding volume. Thus, by reducing the number of ray-primitive intersection tests which may be necessary, the use of a spatial index greatly increases the performance of a ray tracing image processing system. Some examples of different spatial index acceleration data structures are octrees, k dimensional Trees (kd-Trees), and binary space partitioning trees (BSP trees). While several different spatial index structures exist, for ease of describing embodiments of the present invention, a kd-Tree will be used in the examples to follow. However, those skilled in the art will readily recognize that embodiments of the invention may be applied to any of the different types of spatial indexes.

A kd-Tree uses axis aligned bounding volumes to partition the entire scene or space into smaller volumes. That is, the kd-Tree may divide a three-dimensional space encompassed by a scene through the use of splitting planes which are parallel to known axes. The splitting planes partition a larger space into smaller bounding volumes. Together the smaller bounding volumes make up the entire space in the scene. The determination to partition (divide) a larger bounding volume into two smaller bounding volumes may be made by the image processing system through the use of a kd-tree construction algorithm.

One criterion for determining when to partition a bounding volume into smaller volumes may be the number of primitives contained within the bounding volume. That is, as long as a bounding volume contains more primitives than a predetermined threshold, the tree construction algorithm may continue to divide volumes by drawing more splitting planes. Another criterion for determining when to partition a bounding volume into smaller volumes may be the amount of space contained within the bounding volume. Furthermore, a decision to continue partitioning the bounding volume may also be based on how many primitives may be intersected by the plane which creates the bounding volume.

The partitioning of the scene may be represented by a binary tree structure made up of nodes, branches and leaves. Each internal node within the tree may represent a relatively large bounding volume, while the node may contain branches to sub-nodes which may represent two relatively smaller partitioned volumes resulting after a partitioning of the relatively large bounding volume by a splitting plane. In an axis-aligned kd-Tree, each internal node may contain only two branches to other nodes. The internal node may contain branches (i.e., pointers) to one or two leaf nodes. A leaf node is a node which is not further sub-divided into smaller volumes and contains pointers to primitives. An internal node may also contain branches to other internal nodes which are further sub-divided. An internal node may also contain the information needed to determine along what axis the splitting plane was drawn and where along the axis the splitting plane was drawn.

Exemplary Bounding Volumes

FIGS. 5A-5C illustrate a two dimensional space to be rendered by an image processing system and a corresponding kd-tree. For simplicity, a two dimensional scene is used to illustrate the building of a kd-Tree, however kd-Trees may also be used to represent three-dimensional scenes. In the two dimensional illustration of FIGS. 5A-5C splitting lines are illustrated instead of splitting planes, and bounding areas are illustrated instead of bounding volumes as would be used in a three-dimensional structure. However, one skilled in the art will quickly recognize that the concepts may easily be applied to a three-dimensional scene containing objects.

FIG. 5A illustrates a two dimensional scene 505 containing primitives 510 to be rendered in the final picture to be displayed on a monitor 510. The largest volume which represents the entire volume of the scene is encompassed by bounding volume 1 (BV₁). In the corresponding kd-Tree this may be represented by the top level node 550, also known as the root or world node. In one embodiment of an image processing system, an image processing system may continue to partition bounding volumes into smaller bounding volumes when the bounding volume contains, for example, more than two primitives. As noted earlier the decision to continue partitioning a bounding volume into smaller bounding volumes may be based on many factors, however for ease of explanation in this example the decision to continue partitioning a bounding volume is based only on the number of primitives. As can be seen in FIG. 5A, BV₁ contains six primitives, therefore kd-Tree construction algorithm may partition BV₁ into smaller bounding volumes.

FIG. 5B illustrates the same two dimensional scene 505 as illustrated in FIG. 5A. However, in FIG. 5B the tree construction algorithm has partitioned BV₁ into two smaller bounding volumes BV₂ and BV₃. The partitioning of BV₁, was accomplished, by drawing a splitting plane SP₁ 515 along the x-axis at point x₁. This partitioning of BV₁ is also reflected in the kd-Tree as the two nodes 555 and 560, corresponding to BV₂ and BV₃ respectively, under the internal or parent node BV₁ 550. The internal node representing BV₁ may now store information such as, but not limited to, pointers to the two nodes beneath BV₁ (e.g., BV₂ and BV₃), along which axis the splitting plane was drawn (e.g., x-axis), and where along the axis the splitting plane was drawn (e.g., at point x₁).

The kd-Tree construction algorithm may continue to partition bounding volume BV₃ because it contains more than the predetermined threshold of primitives (e.g., more than two primitives). However, the kd-Tree construction algorithm may not continue to partition bounding volume BV₂, because bounding volume BV₂ contains less than or equal to the number of primitives (e.g., only two primitives 510 _(A)). Nodes which are not partitioned or sub-divided any further, such as BV₂, are referred to as leaf nodes.

FIG. 5C illustrates the same two dimensional scene 505 as illustrated in FIG. 5B. However, in FIG. 5C the kd-Tree construction algorithm has partitioned BV₃ into two smaller bounding volumes BV₄ and BV₅. The kd-construction algorithm has partitioned BV₃ using a partitioning plane along the y-axis at point y₁. Since BV₃ has been partitioned into two sub-nodes it may now be referred to as an internal node. The partitioning of BV₃ is also reflected in the kd-Tree as the two leaf nodes 565 and 570, corresponding to BV₄ and BV₅ respectively. BV₄ and BV₅ are leaf nodes because the volumes they represent are not further divided into smaller bounding volumes. The two leaf nodes, BV₄ and BV₅, are located under the internal node BV₃ which represents the bounding volume which was partitioned in the kd-Tree.

The internal node representing BV₃ may store information such as, but not limited to, pointers to the two leaf nodes (i.e., BV₄ and BV₅), along which axis the splitting plane was drawn (i.e., y-axis), and where along the axis the splitting plane was drawn (i.e., at point y₁).

The kd-Tree construction algorithm may now stop partitioning the bounding volumes because all bounding volumes located within the scene contain less than or equal to the maximum predetermined number of primitives which may be enclosed within a bounding volume. The leaf nodes may contain pointers to the primitives which are enclosed within the bounding volumes each leaf represents. For example, leaf node BV₂ may contain pointers to primitives 510 _(A), leaf node BV₄ may contain pointers to primitives 510 _(B), and leaf node BV₅ may contain pointers to primitives 510 _(C).

The resulting kd-Tree structure, or other spatial index structure, may be stored in the shared memory cache 110. The kd-Tree and the size of corresponding data which comprises the kd-Tree may be optimized for storage in the shared memory cache 110.

Iterative Ray Tracing Algorithm

According to one embodiment of the invention, transforming the ray tracing algorithm from a recursive algorithm into an iterative algorithm may enable efficient distribution of workload related to ray tracing amongst a plurality of processing elements. An iterative ray tracing algorithm, in contrast to a recursive ray tracing algorithm, may allow separate processing elements to perform operations relating to determining the color of a single pixel and allow efficient use of processor resources (e.g., memory cache). Efficient distribution of workload amongst a plurality of processing elements may improve ray tracing image processing system performance.

An algorithm for performing ray tracing may be recursive in the sense that it issues an original ray into a three dimensional scene and finishes all ray tracing operations relating to the issued original ray (e.g., traces all secondary rays and performs all ray-object intersection tests) before issuing a subsequent original ray into the three dimensional scene.

For example, an image processing system may use a recursive ray tracing algorithm to render a two dimensional image from a three dimensional scene. The image processing system using a recursive ray tracing algorithm may use a processing element to perform ray tracing. The processor may be used to traverse a ray through a spatial index, and to determine if the ray intersects any objects within a bounding volume of the spatial index. If the ray intersects an object contained within a bounding volume, the image processing system, using the same processor, may issue secondary rays into the three dimensional scene to determine if they intersect any objects and, consequently, contribute color to the object intersected by the original ray. While performing operations related to determining if the secondary rays intersect objects within the three dimensional scene, the processor may store information defining the original ray in the processor's memory cache.

If the processing element determines that the secondary rays intersect objects within the three dimensional scene the image processing element may issue more secondary rays into the scene to determine if those secondary rays intersect objects and contribute color to the object intersected by the original ray. When performing calculations to determine if the secondary rays intersect objects within the three dimensional scene, the processor may store previous secondary ray information in the processor's memory cache. By issuing more and more secondary rays into the scene, the image processing system may finally determine the total contribution of color from secondary rays to the object intersected by the original ray. From the color of the object intersected by the original ray and the contribution of color due to secondary rays, the color of the pixel through which the original ray passed may be finally determined.

Although the recursive ray tracing algorithm determines the color of the pixel through which the original ray passed, each time the image processing system issues more secondary rays into the three dimensional scene, the recursive ray tracing image processing system places information which defines the previous rays (e.g., the original ray or previous secondary rays) into the memory cache of the processing element. The image processing system may store ray information in the cache in order to free registers which may be necessary to perform the calculations related to determining if the subsequent secondary rays intersect objects within the three dimensional scene. Consequently, the recursive ray tracing image processing system may place a large (relative to the size of the cache) amount of information into the processors memory cache for a single pixel.

By storing large amounts of ray information in the memory cache of the processor, there is little or no space in the processor's memory cache for information which defines the objects within the three dimensional scene (i.e., object geometry data). This information may need to be frequently fetched from main memory into the memory cache in order to perform operations to determine if the original or secondary rays intersect objects within the three dimensional scene (thereby “thrashing” the cache). Therefore, the limits of an image processing system which uses the recursive ray tracing technique may be limited by the access time to fetch information from main memory and place it in the processor's memory cache.

However, according to embodiments of the invention, the ray tracing algorithm may be partitioned into an iterative ray tracing algorithm. The iterative ray tracing algorithm may allow separate processing elements to perform portions of the ray tracing algorithm. By allowing separate processing elements to perform portions of the ray tracing algorithm, the amount of information which needs to be cached (e.g., original rays and secondary rays) may be reduced. Furthermore, according to embodiments of the invention, the iterative ray tracing algorithm may be used in conjunction with the low-latency high-bandwidth communications network and the shared memory cache 110 in order improve the performance of a ray tracing image processing system.

The low-latency high-bandwidth communications network of inboxes, as described above with regards to FIGS. 3A-3C, may be used to pass or send data processing information (e.g., information defining original rays and secondary rays) which has little use when tracing subsequent rays or rendering subsequent frames, according to embodiments of the invention. In addition, according to embodiments of the invention, the ray tracing image processing system may use a shared coherent memory cache to store information which may be used by the image processing system when tracing subsequent rays or performing ray tracing for a subsequent frame.

FIG. 6 is a flowchart which illustrates a partitioned and thus iterative ray tracing algorithm or method 600 which may be used in a multi processor image processing system, according to one embodiment of the invention. The method 600 begins at step 605 when the image processing system issues an original ray into the three dimensional scene. The original ray may pass through a pixel as it traverses into the three dimensional scene. The original ray may be used to determine the color of the pixel through which the original ray passed.

Next, at step 610 the image processing system may use a use a workload manager 205 processing element to traverse the spatial index (e.g., kd-Tree). The spatial index may be stored within the shared memory cache 110 of the image processing system. Traversing the kd-Tree may include performing calculations which determine if the original ray intersects bounding volumes which are defined by nodes within the spatial index. Furthermore, traversing the spatial index may include taking branches to nodes which defined bounding volumes intersected by the ray. A workload manager 205 may use the coordinates and trajectory of an issued ray (e.g., the original ray) to determine if the ray intersects bounding volumes defined by the nodes in the spatial index. The workload manager 205 may continue traversing the spatial index until the original ray intersects a bounding volume which contains only primitives (i.e., a leaf node).

At step 615, after the workload manager 205 has traversed the original ray to a leaf node, the workload manager 205 may send the original ray and information which defines the leaf node to a vector throughput engine 210. The workload manager 205 may send information which defines the original ray and the leaf node (e.g., trajectory of the ray, pixel through which the original ray passed, bounding volume defined by the leaf node, etc.) to the vector throughput engine 210. The workload manager 205 may send the information to the vector throughput engine 210 by writing the information defining the ray and the intersected leaf node to the inbox of the vector throughput engine 210.

By coupling the pixel information with the information which defines the original ray, there is no need to send the original ray back to the workload manager 205 if the vector throughput engine 210 determines that the ray intersected an object and, consequently, determines a color of the pixel. According to one embodiment of the invention, the vector throughput engine 210 may use the pixel information to update the color of the pixel by writing to memory location within a frame buffer (e.g., stored in the shared memory cache 110) which corresponds to the pixel. By updating the pixel color as secondary rays intersect objects within the three-dimensional scene, the number of rays relating to the same pixel that need to be stored (e.g., in cache memory) may be reduced.

After the workload manager 205 sends the original ray information to the vector throughput engine 210, the image processing system may issue a subsequent original ray into the three dimensional scene. The workload manager 205 may immediately begin traversing this subsequently issued original ray through the spatial index after the workload manager 205 has sent the original ray to a vector throughput engine 210. Thus, the workload manager 205 may be continuously traversing rays through the spatial index, rather than wait until the determination of whether the original ray intersected an object is complete, as in a recursive ray tracing algorithm. Furthermore, the workload manager 205 may be traversing rays through the spatial index as the vector throughput engine 210 is determining if previously issued rays intersect objects within the bounding volumes defined by leaf nodes. According to one embodiment of the invention, vector throughput engines 210 may be responsible for performing ray-primitive intersection tests. That is, the vector throughput engines 210 may determine if a ray intersects any primitives contained within the bounding volume defined by the leaf node.

Therefore, at step 620, a vector throughput engine 210 that receives the ray and leaf node information in its inbox may perform ray-primitive intersection tests to determine if the ray intersects any primitives within the bounding volume defined by the leaf node. The geometry which defines the primitives may be stored within the shared memory cache 110, and thus may not need to be fetched from main memory. By storing the geometry for primitives in the shared memory cache 110, the iterative ray tracing algorithm may not need to fetch the geometry from main memory as is the case with the recursive ray tracing algorithm. If the vector throughput engine 210 determines that the original ray intersected a primitive contained within the bounding volume defined by the leaf node, the vector throughput engine 210 may proceed to step 630.

At step 630, the vector throughput engine 210 may determine the color of the intersected primitive at the point which the original ray intersected the primitive. For example, the color of the primitive may be stored in the shared memory cache 110 and the vector throughput engine 210 may read the color information from the shared memory cache 210.

After determining the color of the primitive at the ray-primitive intersection point, the vector throughput engine 210 may update the color of pixel through which the ray passed. This may be accomplished, for example, by writing to a memory location within a frame buffer which corresponds to the pixel through which the original ray passed. By updating the pixel information as a ray-primitive intersection is determined and before determining the color contributions for all secondary rays relating to a original ray, the amount of information which may need to be stored in a memory cache may be reduced. In contrast, a recursive ray tracing algorithm may not store the color of the pixel in a frame buffer until all color contributions from secondary rays have been determined, which increases the amount of information which may need to be stored in a processor's memory cache.

After updating the pixel color, the vector throughput engine 210 may proceed to step 635, where, the vector throughput engine 210 may generate secondary rays. As described previously with regards to FIG. 4, a ray tracing image processing system may use secondary rays determine additional color contribution to the intersected object and thus to the pixel through which the original ray passed. Secondary rays may be, for example, reflected rays, transmitted (refracted) rays, or shadow rays. Generating secondary rays may include, for example, determining the trajectory of the secondary rays based on the trajectory of the original ray, surface properties of the intersected object, and an angle of intersection of the original ray with the intersected object.

After generating secondary rays, the vector throughput engine 210, at step 640 may send the secondary rays to a workload manager 205. The vector throughput engine 210 may send the secondary rays to a workload manager 205 by placing the information which defines the secondary rays (e.g., trajectory, information defining the pixel through which the original ray passed, etc.) in an inbox 115 of a workload manager 205. According to one embodiment of the invention, the vector throughput engine 210 may send the secondary rays to the workload manager 205 which traversed the original ray through the spatial index. However, according to another embodiment of the invention, the image processing system may contain a plurality of workload managers and the vector throughput engine 210 may send the secondary rays to a workload manager which did not traverse the original ray through the spatial index.

After sending the secondary rays to a workload manager 205, the vector throughput engine 210 may retrieve other information defining rays from an inbox which may be waiting to have ray-primitive intersection tests performed. The rays waiting in the vector throughput engine's 210 inbox may have been previously traversed through a spatial index by a workload manager 205. Therefore, the vector throughput engine 210 may perform more ray-primitive intersection tests to determine if rays (i.e., original or secondary) intersect objects within bounding volumes defined by leaf nodes. Thus, the vector throughput engine 210 may continuously perform operations related to ray-primitive intersection tests, determining primitive colors, updating pixel colors, and generating secondary rays.

After receiving a secondary ray from a vector throughput engine 210, a workload manager 205 may execute steps 610 and 615, as described above, to determine if the secondary ray intersects a leaf node.

Returning to step 625, if the vector throughput engine 210 determines that the ray did not intersect a primitive contained within bounding volume defined by the leaf node, the vector throughput engine 210 may assign the pixel through which the original ray passed a background color of the three-dimensional scene. The background color may be assigned to the pixel because the original ray did not intersect any primitives contained within the three dimensional scene. However, according to other embodiments of the invention, if the ray did not intersect any primitives contained within the leaf-node bounding volume, the vector throughput engine 210 may send the ray back to a workload manager 205 such that the workload manager 205 may traverse the ray through the spatial index again to determine if the ray intersected any other leaf nodes containing primitives.

Exemplary Use of an Iterative Ray Tracing Algorithm

FIG. 7 illustrates exemplary rays issued from an image processing system into a three dimensional scene 505, according to one embodiment of the invention. For clarity, the three dimensional scene 505 is the same as the three-dimensional scene used in FIGS. 5A-5C to illustrate the construction of a kd-tree. Therefore, the kd-tree which corresponds to the three dimensional scene 505 is the same as the kd-tree which was constructed with regards FIGS. 5A-5C. As illustrated in FIG. 7, a viewer 705 represents the origin of a plurality of original rays 710 ₁₋₄ which may be issued into the three dimensional scene 505 by the image processing system. As each original ray 710 ₁₋₄ is issued into the three-dimensional scene, the original rays may first pass through a corresponding pixel in a grid (frame) of pixels 715. Although only four pixels 715 and four original rays 710 ₁₋₄ are illustrated in FIG. 7, to render a final two dimensional image from a three dimensional scene many more pixels may be necessary, and many more original rays may be issued.

A first original ray 710 ₁ may be issued by the image processing system and pass through a first pixel 715 ₁. The first original ray 710 ₁ may intersect bounding volume 4 (BV₄) at an intersection point I₁. To facilitate understanding, the image processing system in this example may follow a pattern of issuing rays starting from the top of the grid of pixels 715 and continue issuing rays, one ray per pixel, moving down the grid of pixels until a ray has been issued for each pixel in the grid of pixels.

A second original ray 710 ₂ and a third original ray 710 ₃ may also be issued by the image processing system which may pass through a second pixel 715 ₂ and a third pixel 715 ₃ respectively. The second original ray 710 ₂ and the third original ray 710 ₃ may also intersect BV₄ at a second intersection point I₂ and a third intersection point I₃, respectively. Thus the first original ray 710 ₁, the second original ray 710 ₂, and the third original ray 710 ₃ all intersect the same bounding volume. Furthermore, a fourth original ray 710 ₄ may be issued by the image processing system and may pass through a fourth pixel 815 ₄. The fourth original ray 710 ₄, in contrast to the first three original rays 710 ₁₋₃, may intersect bounding volume 5 (BV₅) at intersection point I₄.

FIG. 8A illustrates the traversal of the first original ray 710 ₁ ray through a spatial index 805 (e.g., a kd-tree). Furthermore, as indicated by the shaded box 205, FIG. 8A illustrates a workload manager 205 performing operations related to the traversal of the first original ray 710 ₁ through the spatial index 805. The workload manager 205 may traverse the ray through the spatial index 805 by taking branches to nodes defining bounding volumes intersected by the ray until a leaf node is reached (as illustrated in FIG. 8A by the darkened branches and nodes). As illustrated in FIG. 7 the original ray 710 ₁ intersects BV₄, therefore, the workload manager 205 will traverse the first original ray 710 ₁ to the leaf node which defines BV₄. After traversing the ray to a leaf node, the workload manager 205 may send the first original ray 710 ₁ (e.g., send information which defines the first original ray 710 ₁ and information which defines the pixel 715 ₁ through which the first original ray passed) and information defining the intersected leaf node (i.e., BV₄) to a vector throughput engine 210.

According to embodiments of the invention, after the workload manager 205 sends the first original ray 710 ₁ to a vector throughput engine 210, the workload manager 205 may begin traversing the second original ray 710 ₂ through the spatial index. Thus, the workload manager 205 may be constantly traversing rays through the spatial index 805 while the vector throughput engines 210 are determining if rays intersect objects within the bounding volumes defined by traversed to leaf nodes.

FIG. 8B illustrates the first original ray 710 ₁ traversing through the bounding volume 4 (BV₄). Furthermore, as indicated by the shaded box, FIG. 8B illustrates the vector throughput engine 210 performing ray-primitive intersection tests after the vector throughput engine has received the information defining the first original ray 710 ₁ and the information defining the bounding volume BV₄. As described with regards to FIG. 6, the vector throughput engine 210 may execute ray-primitive intersection tests to determine if the original ray 710 ₁ intersects primitives contained within the bounding volume BV₄.

The vector throughput engine 210 may perform tests with the first original ray 710 ₁ against a first object 720 within the bounding volume BV₄, and against a second object 725 within the bounding volume BV₄. As illustrated in FIG. 8B, the vector throughput engine 210 may determine that the first original ray 710 ₁ intersects the first object 720.

As described previously with respect to method 600, after determining that the first original ray 710 ₁ intersects an object, the vector throughput engine 210 may determine the color of the first object 720 at the point which the first original ray 710 ₁ intersected the first object 720. After determining the color of the object 720 at the intersection point, the vector throughput engine 210 may update the color of the pixel 715 ₁ through which the first original ray 710 ₁ passed (e.g., by writing to a frame buffer memory location which corresponds to the pixel 715 ₁).

After determining the color of the object 720 at the intersection point, the vector throughput engine 210 may generate secondary rays. For example, as illustrated in FIG. 8C the vector throughput engine 210 may generate a reflected ray 730 and a transmitted (refracted) ray 735. Both secondary rays (730 and 735) originate from the point where the first original ray 710 ₁ intersected the object 720. As described above, the secondary rays may be used to determine additional color contribution to the object at the point which the first original ray 710 ₁ intersected the object 720. The generation of the secondary rays may include determining a trajectory for each secondary ray and tagging the secondary ray such that the additional color contribution from the secondary ray may be used to update the color of the pixel 715 ₁ through which the first original ray 710 ₁ passed.

After generating the secondary rays (730 and 735), the vector throughput engine 210 may send the secondary rays (730 and 735), via an inbox, to a workload manager 205. A workload manager 205 which receives the secondary rays (730 and 735) may use the information which defines the secondary rays (i.e., trajectory of secondary rays) to traverse the spatial index 805. For example, the shaded box in FIG. 8D illustrates a workload manager 205 which may traverse the spatial index 805 with a secondary ray (e.g., 730) which was generated by a vector throughput engine 210. The workload manager 205 may traverse the secondary ray to a leaf node. After the secondary ray has been traversed to a leaf node, the workload manager 205 may send the secondary ray and information defining the bounding volume intersected by the secondary ray to a vector throughput engine 210 to determine if the secondary ray intersects any objects with the bounding volume intersected by the secondary ray.

As the vector throughput engines 210 determine that the original ray or secondary rays strike objects within the three dimensional scene, the color of the pixel through which the original ray passed may be updated within the frame buffer. According to embodiments of the invention, all secondary rays relating to an original ray, and thus to the pixel through which the original ray passed, may be traced through the three dimensional scene and their color contributions saved in the frame buffer to determine the final color of the pixel. However, according to other embodiments of the invention, a finite number of secondary rays relating to the original ray may be traced through the three dimensional scene to determine the color of the pixel. By limiting the number of secondary rays which are traced through the three dimensional scene and thus contribute to the color of the pixel, the amount of processing necessary to determine a final color of the pixel may be reduced.

Spacial Index Mapping to Processor Thread Resources

As described above, the vector throughput engine (VTE) 210 may be responsible for operations such as performing ray-primitive intersection tests, updating pixel color and issuing secondary rays. As illustrated in FIG. 2, each multiple core processing element may contain a plurality of VTEs 210 and each VTE 210 may contain a plurality of processing threads (VTE threads). When tracing rays to render a two-dimensional image from a three-dimensional scene many (thousands if not millions) of rays may be traced to render a signal image resulting in many VTE operations needing to be performed. Furthermore, the image processing system may be part of a larger system (e.g., a video game console, etc.) which may require frames to be rendered many times per second (e.g., 60 frames per second). In order to perform a sufficient number of VTE operations within the required timeframe, the image processing system may distribute VTE workload amongst the plurality of VTEs and VTE threads.

According to one embodiment of the invention, the image processing system may distribute workload amongst the VTE threads by assigning a number of the spatial index leaf nodes to each VTE thread.

For example, a VTE 210 may contain four VTE threads. Each VTE thread in the image processing system may be identified by a VTE thread id tag. For example, the a first VTE thread may have a VTE thread id tag of 0000, a second VTE thread may have a thread id tag of 0001, a third VTE thread with a thread id tag of 0010, and a fourth VTE thread with a thread id tag of 0011. Each of these four threads may be responsible for a number of spatial index leaf nodes. Although the image processing system herein is described as having four VTE threads, in various embodiments of the invention the image processing system may contain more or less VTE threads.

FIG. 9 illustrates an exemplary mapping of leaf nodes of the spatial index 900 to VTE threads in order to distribute workload in an image processing system, according to one embodiment of the invention. The spatial index 900 may be used by the image processing system to perform ray tracing. The spatial index 900 contains a world node, a plurality of internal nodes, and a plurality of leaf nodes.

The mapping of leaf nodes to VTE threads is illustrated in FIG. 9 by the curved regions surrounding leaf nodes of the spatial index 900. As illustrated in FIG. 9, the first VTE thread 0000 may be responsible for VTE operations relating to a first group of nine leaf nodes, the second processing thread 0001 may be responsible for a second group of nine leaf nodes, the third processing thread 0010 may be responsible for a third group of nine leaf nodes, and the fourth processing thread 0011 may be responsible for a fourth group of nine leaf nodes.

The leaf nodes may be mapped to each VTE thread such that workload experienced by the VTE threads is relatively balanced. According to one embodiment of the invention, balanced workload may be accomplished by mapping a relatively equal number of leaf nodes to VTE threads. As illustrated in FIG. 9, each processing thread may be responsible for performing VTE operations for an equal number of leaf nodes (9).

According to another embodiment of the invention, the distribution of workload amongst VTE threads may be accomplished by balancing the number of primitives for which each VTE thread is responsible. For example, a first VTE thread may be responsible for a smaller number of leaf nodes than a second VTE thread; however, the total number of primitives contained within the leaf nodes for which each VTE thread is responsible may be relatively equal.

FIG. 10 is a flowchart illustrating an exemplary method 1000 of performing ray tracing in an image processing system containing a plurality of VTE threads, according to one embodiment of the invention. The method 1000 demonstrates an exemplary method of identifying the responsible VTE thread for a traversed to leaf node in an image processing system which has mapped leaf nodes to threads. Furthermore, the method 1000 demonstrates an exemplary redistribution of VTE workload to idle VTE threads.

The method 1000 may begin at step 1005 where a workload manager 205 may traverse a ray through the spatial index 900 until a leaf node is reached. The workload manager 205 may traverse the ray by taking branches to nodes which define bounding volumes intersected by the ray. Once the workload manager 205 traverses the ray to a leaf node, the workload manager 205 may proceed to step 1010.

At step 1010, the workload manager 205 may determine the VTE thread which is responsible for performing VTE operations for the traversed to leaf node. According to one embodiment of the invention, the thread which is responsible for the performing VTE operations for a leaf node may be identified within the spatial index. For example, the spatial index may contain information identifying the responsible VTE thread with the information which defines the leaf node (e.g., a set of bits stored in the leaf nodes of the spatial index identifying the responsible VTE thread). However, according to another embodiment of the invention, the VTE thread which is responsible for performing VTE operations for the traversed to leaf node may be identified in a thread table.

For example, FIG. 11 illustrates a thread table 1100. The thread table 1100 may have a row corresponding to each leaf node in the spatial index. Furthermore, the thread table 1100 may list the VTE thread which is responsible for performing VTE operations with regards to each leaf node. Therefore, a workload manager 205 may determine the responsible VTE thread by searching for the entry within the thread table 1100 which contains the traversed to leaf node. In addition, as will be described in further detail below, the thread table 1100 may also contain a column which indicates a status of each VTE thread (e.g., idle). The thread table 1100 may be stored, for example, in the main memory of the image processing system or in cache memory 230 of a multiple core processing element 2201.

After determining the VTE thread which may be responsible for the traversed to leaf node, at step 1015 the workload manager 205 may send information defining the ray (e.g., the trajectory of the ray) and information defining the traversed to leaf node (e.g., geometry of the bounding volume which the leaf node defines) to the responsible VTE thread. The workload manager 205 may send the information to the responsible VTE thread, for example, by placing the ray and leaf node information in an inbox (e.g., inbox 302) corresponding to the responsible VTE thread.

Next, at step 1020, the VTE thread may check its inbox for ray and leaf node information. After checking for rays in step 1020, at step 1025 a determination is made whether or not the VTE thread has received ray information. If so, the VTE thread may proceed to step 1030. At step 1030, the VTE thread may determine if the number of rays present in its inbox exceeds a threshold indicating a desired threshold limit on workload.

The threshold may be used to determine if it is more efficient for the VTE thread to perform the VTE operations itself, or if it may be more efficient for the VTE to send the received ray to another (e.g., idle) VTE thread to perform the VTE operations. It may be more efficient to send the ray to another VTE thread if the VTE currently has workload while another VTE may not have workload (or may have less workload) and, thus, may be idle (or less busy).

For example, the threshold may be set such that the time necessary to determine if there is an available VTE thread and send the ray to the available VTE thread does not exceed the time for the initial VTE to perform the VTE operations itself. If the threshold is not exceeded, the initial VTE thread may proceed to step 1045 and perform VTE operations with the ray and leaf node information sent by the workload manager 205.

However, if the number of rays in the VTE thread's inbox exceeds the threshold, the VTE thread may proceed to step 1035 where the VTE thread may search for another VTE thread which may be available to perform the VTE operations. According to one embodiment of the invention, an available VTE thread may be, for example, a VTE thread which is idle (i.e., currently not performing VTE operations). The initial VTE thread may determine which VTE thread or threads are idle by checking, for example, the thread table. A VTE thread may search for an idle VTE thread by examining the idle bits of VTE threads in the thread table 1100.

After searching the thread table 1100, at step 1040, the VTE thread may determine if it found a VTE thread which is idle. If the VTE thread did not find an idle VTE thread, the original VTE thread may perform the VTE operations on the ray itself.

However, if the VTE thread which received the ray first found an idle VTE thread, the VTE thread which received the ray first may proceed to step 1050 and send the ray and leaf node information to the VTE thread which is idle. For example, a workload manager 205 may traverse a ray to a leaf node identified by the leaf node id 0000. The workload manager 205 may determine from the thread table 1100 that VTE thread 0000 is responsible for performing VTE operations for leaf node 0000. Therefore, the workload manager 205 may send the ray and leaf node information to VTE thread 0000. However, VTE thread 0000 may determine at step 1030 that the number of rays in its inbox needing VTE operations exceeds the threshold. Therefore, VTE thread 0000 may access the thread table 1100 to determine if another VTE thread in the image processing system is idle, and therefore able to perform VTE operations with the ray traversed to leaf node 0000. By examining the idle bits in the thread table 1100, the VTE thread 0000 may determine that VTE thread 0011 is idle. Therefore, VTE thread 0000 may send the ray to VTE thread 0011.

By transferring rays from a VTE thread which is busy performing VTE operations to a thread which is idle, more efficient use of image processing system resources may be achieved. More efficient use of image processing system resources may lead to a reduction in time necessary to render a two dimensional image from a three-dimensional scene and thus improve the performance of the image processing system.

After sending the ray to an idle VTE thread, the VTE thread may proceed to step 1045 to perform VTE operations on the rays which are remaining in its inbox. After performing VTE operations on the rays remaining in its inbox, the VTE thread may proceed to step 1020 to check for more rays in its inbox. If at step 1025 the VTE thread determines that there are no rays in its inbox, the VTE thread may proceed to step 1055 where the VTE thread may assert its idle bit in the thread table 1100. By asserting its idle bit in the thread table 1100, the VTE is signaling to other VTE threads in the image processing system that it is available to perform VTE operations.

Spatial Index Remapping to Processor Thread Resources

According to embodiments of the invention, the image processing system may periodically remap leaf nodes to different VTE thread resources in the spatial index. Remapping leaf nodes to VTE threads may be performed in response to a detection of unbalanced workload experienced by VTE threads. The image processing system may monitor the workload experienced by all VTE threads, and, in response to the monitored workload, remap the leaf nodes for which each VTE thread is responsible. Remapping leaf nodes to VTE threads to balance workload may improve the performance of the image processing system. Balanced workload may improve the performance of the image processing system by ensuring that one VTE thread (or a plurality of VTE threads) is (are) not performing the majority of VTE operations while other VTE threads are idle.

The remapping interval may vary widely for different embodiments. For example, the remapping interval may vary from frequent (e.g., multiple timers per frame) to relatively infrequent (e.g., once per ‘N’ frames wherein N may be very large). For some embodiments, the remapping interval may be determined by the ratio of load imbalance drift (which may depend on the scene data and user input) to the cost overhead of the remapping itself. This overhead may include the necessary performance measurements (to measure loads), thread swapping, and the like.

According to embodiments of the invention, VTE threads which are experiencing a higher workload may be responsible for less leaf nodes after remapping. Conversely, VTE threads which are experiencing a relatively lower workload may be responsible for more leaf nodes after remapping.

FIG. 12 is a flowchart illustrating an exemplary method 1200 of performing ray tracing and remapping leaf nodes to VTE threads, according to one embodiment of the invention. The method 1200 begins at step 1205 where the image processing system may create an initial mapping of leaf nodes to VTE threads. The leaf node to VTE thread mapping may be, for example, stored in the thread table 1100 as illustrated in FIG. 11.

As illustrated in FIG. 9, the image processing system initially may map four groups or regions of leaf nodes to three VTE threads (e.g., VTE threads 0000, 0001, 0010, and 0011). After mapping the leaf nodes to VTE threads, at step 1210 the image processing system may perform ray tracing for a frame (e.g., frame N).

After rendering the frame (or during the rendering of the frame), the image processing system may monitor the workload experienced by the VTE threads at step 1215. The image processing system may determine the workload presented to the VTE threads by monitoring various performance parameters. For example, the workload presented to a VTE thread may be determined by examining a performance counter for the thread. A performance counter may compile data relating to performance events within a processing element (e.g., count the number of rays processed by a VTE thread).

The workload experienced by a VTE thread may also be determined by monitoring the traffic through the inboxes 115 corresponding to a VTE thread.

Next, at step 1220, the image processing system may determine if the workload experienced by the VTE threads is balanced. To determine if the workload experienced by the VTE threads is balanced the image processing system may compare the workload experienced by each VTE thread. A high performance counter value or a high amount of inbox traffic relative to other VTE threads may indicate that a VTE thread is experiencing an increased workload relative to other VTE threads.

A VTE thread may experience an increased workload relative to other VTE threads, for example, if the VTE thread is responsible for more leaf nodes than other VTE threads. Furthermore, a VTE thread may experience more workload relative to other VTE threads, for example, if the VTE thread is responsible for leaf nodes which together contain more primitives relative to the number of primitives which other VTE threads are responsible.

If the workload experienced by VTE threads is balanced, the image processing system may return to step 1210 to render a subsequent frame (e.g., frame N+1) from the three-dimensional scene. However, if the workload experienced by the VTE threads is not balanced, the image processing system may proceed to step 1225. At step 1225 the image processing system may remap the leaf nodes to VTE threads in an effort to balance the workload experienced by the VTE threads. The image processing system may remap the leaf nodes to VTE threads corresponding to the workload monitored in step 1215.

After remapping the leaf nodes to other VTE threads, the image processing system may return to step 1210 of method 1200 to render a new frame (e.g., frame N+1) from the three-dimensional scene.

For example, in step 1215, the image processing system may determine that the workload experienced by the VTE threads (e.g., threads 0000, 0001, 0010, and 0011) is unbalanced when performing VTE operations relating to the leaf nodes illustrated in the spatial index 900 in FIG. 9. Specifically, the image processing system may determine that the VTE thread 0010 is experiencing a larger workload relative to the VTE thread 0011. In response the unbalanced workload being experienced by the VTE threads, according to one embodiment of the invention, the image processing system may remap leaf nodes in the spatial index 900 to other VTE threads.

For example, in response to VTE thread 0010 experiencing a relatively larger workload than VTE thread 0011, the image processing system may remap a number of the leaf nodes (2) for which VTE thread 0010 was responsible to VTE thread 0011.

The spatial index 900 is illustrated in FIG. 13 with a new mapping of leaf nodes to VTE threads. For example, for the next frame to be rendered (i.e., frame N+1) thread 0011 may be responsible for a larger number of leaf nodes (11) in contrast to the number of leaf nodes VTE thread 0011 was responsible for the previous frame (9). Furthermore, in contrast to the number of leaf nodes (9) which thread 0010 was responsible for during the rendering of the previous frame, thread 0010 may be responsible for a smaller number of leaf nodes (7) while rendering next frame.

By remapping the leaf nodes for which each VTE thread is responsible, balanced workload may be experienced by all of the vector throughput engines in the image processing system. Balanced workload experienced by the VTE threads may improve the performance of the ray tracing image processing system.

Conclusion

By mapping leaf nodes to VTE threads, efficient distribution of workload in an image processing system may be achieved. In addition, VTE threads may use a thread table to redistribute workload from VTE threads which are experiencing an increased workload to VTE threads which may be idle. Furthermore, the workload experienced by VTE threads may be monitored in order to determine if the workload is balanced across all VTE threads. Periodically the leaf nodes may be remapped to other VTE threads in response to a detected imbalance in workload between VTE threads. By monitoring the workload experienced by the VTE threads and remapping leaf nodes to different VTE threads in response to unbalanced workload, efficient distribution of workload may be maintained. Efficient distribution of workload may improve the performance of the image processing system.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

1. A method of distributing workload in a ray tracing image processing system, comprising: providing a spatial index having internal nodes and leaf nodes defining bounded volumes of a three dimensional scene; dividing the leaf nodes into at least a first group and a second group, wherein a first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group, and wherein a second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; tracing rays to render a two dimensional image from the three dimensional scene; monitoring at least a workload experienced by the first processing element or a workload experienced by the second processing element; and re-dividing the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.
 2. The method of claim 1, wherein tracing rays to render a two dimensional image from the three dimensional scene comprises: issuing at least a first ray into the three dimensional scene; traversing the spatial index by taking branches to nodes defining bounding volumes intersected by the first ray until a first leaf node is reached; determining if the first leaf node is in at least one of the first group or the second group; and based on the determination, sending information defining the first ray and information defining the first leaf node to at least one of the first processing element or the second processing element.
 3. The method of claim 1, wherein if the workload experienced by the first processing element is greater than the workload experienced by the second processing element, re-dividing the leaf nodes such that the new first group of leaf nodes is smaller than the new second group of leaf nodes; and wherein if the workload experienced by the second processing element is greater than the workload experienced by the first processing element, re-dividing the leaf nodes such that the new second group of leaf nodes is smaller than the new first group of leaf nodes.
 4. The method of claim 3, wherein the first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new first group, and wherein the second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new second group.
 5. The method of claim 1, wherein monitoring a workload comprises: monitoring a performance counter for at least one of the first processing element or the second processing element.
 6. The method of claim 5, wherein a performance counter counts the number of ray-primitive intersection tests performed by at least one of the first processing element or the second processing element.
 7. The method of claim 1, wherein monitoring a workload comprises: monitoring an inbox associated with at least one of the first processing element or the second processing element.
 8. A computer readable medium containing a program which, when executed, performs an operation, comprising: providing a spatial index having internal nodes and leaf nodes defining bounded volumes of a three dimensional scene; dividing the leaf nodes into at least a first group and a second group, wherein a first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group, and wherein a second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; tracing rays to render a two dimensional image from the three dimensional scene; monitoring at least a workload experienced by the first processing element or a workload experienced by the second processing element; and re-dividing the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.
 9. The computer readable medium of claim 8, wherein tracing rays to render a two dimensional image from the three dimensional scene comprises: issuing at least a first ray into the three dimensional scene; traversing the spatial index by taking branches to nodes defining bounding volumes intersected by the first ray until a first leaf node is reached; determining if the first leaf node is in at least one of the first group or the second group; and based on the determination, sending information defining the first ray and information defining the first leaf node to at least one of the first processing element or the second processing element.
 10. The computer readable medium of claim 8, wherein if the workload experienced by the first processing element is greater than the workload experienced by the second processing element, re-dividing the leaf nodes such that the new first group of leaf nodes is smaller than the new second group of leaf nodes; and wherein if the workload experienced by the second processing element is greater than the workload experienced by the first processing element, re-dividing the leaf nodes such that the new second group of leaf nodes is smaller than the new first group of leaf nodes.
 11. The computer readable medium of claim 10, wherein the first processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new first group, and wherein the second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new second group.
 12. The computer readable medium of claim 8, wherein monitoring a workload comprises: monitoring a performance counter for at least one of the first processing element or the second processing element.
 13. The computer readable medium of claim 12, wherein a performance counter counts the number of ray-primitive intersection tests performed by at least one of the first processing element or the second processing element.
 14. The computer readable medium of claim 8, wherein monitoring a workload comprises: monitoring an inbox associated with at least one of the first processing element or the second processing element.
 15. An image processing system, comprising: a spatial index having internal nodes and leaf nodes defining bounding volumes of a three-dimensional scene; a first processing element; and a second processing element; and wherein the image processing system is configured to divide the leaf nodes of the spatial index into at least a first group and a second group, and trace rays to render a two dimensional image from the three dimensional scene; wherein the first processing element configured to perform operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the first group; wherein the second processing element configured to perform operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the second group; and wherein the image processing system is further configured to monitor at least a workload experienced by the first processing element or a workload experienced by the second processing element, and re-divide the leaf nodes into at least a new first group and a new second group, wherein the number of leaf nodes within at least one of the new first group or the new second group is based on at least one of the workload experienced by the first processing element or the workload experienced by the second processing element.
 16. The image processing system of claim 15, wherein tracing rays to render a two dimensional image the three dimensional scene comprises: the image processing system configured to issue at least a first ray into the three dimensional scene, traverse the spatial index by taking branches to nodes defining bounding volumes intersected by the first ray until a first leaf node is reached, determine if the first leaf node is in at least one of the first group or the second group, and, based on the determination, send information defining the first ray and information defining the first leaf node to at least one of the first processing element or the second processing element.
 17. The image processing system of claim 15, wherein if the workload experienced by the first processing element is greater than the workload experienced by the second processing element, the image processing system is configured to re-divide the leaf nodes such that the new first group of leaf nodes is smaller than the new second group of leaf nodes; and wherein if the workload experienced by the second processing element is greater than the workload experienced by the first processing element, the image processing system is configured to re-divide the leaf nodes such that the new second group of leaf nodes is smaller than the new first group of leaf nodes.
 18. The image processing system of claim 17, wherein the first processing element is configured to perform operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new first group, and wherein the second processing element performs operations to determine if rays intersect primitives contained within bounding volumes defined by leaf nodes in the new second group.
 19. The image processing system of claim 15, further comprising: a performance counter for counting the number of ray-primitive intersection tests performed by at least one of the first processing element or the second processing element; and wherein monitor at least a workload experienced by the first processing element or a workload experienced by the second processing element comprises: monitoring a performance counter for at least one of the first processing element or the second processing element.
 20. The image processing system of claim 15, further comprising: an inbox associated with at least one of the first processing element or the second processing element; and wherein monitor at least a workload experienced by the first processing element or a workload experienced by the second processing element comprises: monitoring an inbox associated with at least one of the first processing element or the second processing element. 